Solder metal, soldering flux and solder paste

ABSTRACT

Solder metal consists essentially of 8.8 to 5.0 mass % of Zn, 0.05 to 0 mass % of Bi and the balance of Sn and unavoidable impurities.

CROSS REFERENCE TO RELATED APPLICATION

This application is an application filed under 35 U.S.C. §111(a)claiming the benefit pursuant to 35 U.S.C. §119(e)(1) of the filing dateof Provisional Application No. 60/354,021 filed Feb. 5, 2002 pursuant to35 U.S.C. §111(b).

TECHNICAL FIELD

The present invention relates to solder metal, soldering flux, solderpaste and similar materials for use in mounting electronic parts on asubstrate.

BACKGROUND ART

In the electronics industry, solder metal, soldering flux, solder pasteand similar materials are used for mounting electronic parts on thesurface of a substrate. Since solder paste is excellent in printingproperty and has good viscosity, it is suitable for automatic mountingand has recently been used in an increasing amount.

By screen-printing or with a dispenser, a solder paste is applied to asubstrate, on which electronic parts are then mounted and to which theelectronic parts are fixed through reflowing of the solder paste. Theterm “reflowing” used herein refers to a sequential process includingthe steps of preheating the substrate on which electronic parts havebeen placed and heating the substrate at a temperature higher than themelting temperature of the solder paste, thereby joining the parts tothe substrate.

Recently, in order to keep pace with the trend for scaling down the sizeof electronic products, demand has arisen for electronic parts with finepitches, and fine-pitch electronic parts, such as 0.3-mm-pitch quad flatpackage (QFP) type LSIs and chip size packages (CSPs), have currentlybeen widely employed. Under these circumstances, solder paste isrequired to have high wettability for joining fine-pitch electronicparts so as to impart high thermal shock resistance to the joinedproducts and to attain proper mounting of electronic parts on asubstrate. In order to meet this demand in the industry, solder metaland solder paste must satisfy the above requirements.

However, when conventional solder alloy or solder paste is used forsoldering fine-pitch or large-scaled electronic parts recently demanded,wettability and thermal shock resistance are unsatisfactorily attained,thus deteriorating reliability of joining portions. In addition, becauseof recent environmental concerns in relation to lead, use of Pb-freesolder is being encouraged and accordingly development of Pb-free solderis under way.

Sn—Ag based, Sn—Cu based, Sn—Bi based and Sn—Zn based solder membershave attracted attention as promising ones, because these Pb-free soldermembers are advantageous in terms of melting temperature, wettabilityand reliability in joining. However, the solder members have drawbacks.Sn—Bi based solder members are fragile though they are advantageous interms of melting temperature. Therefore, Sn—Bi based solder has not beenused in practice. Since Sn—Cu based and Sn—Ag based solder members haveexcessively high melting temperatures, they when used raise a problemthat electronic parts to be soldered are affected during soldering.Sn—Ag based solder members cost more than conventionally used Sn—Pbsolder because they contain expensive Ag. In addition, excessive use ofAg results in depletion of resources and, when released to theenvironment, might affect the ecological system in view of life cycleassessment (LCA).

Sn—Zn based solder has attracted attention as promising Pb-free solder,because the solder has a melting temperature most similar to that ofconventional Sn—Pb eutectic alloy. To Sn—Zn based solder, Bi is added inorder to further lower the melting temperature thereof and improvewettability. However, a recent study has revealed that addition of Bi toSn—Zn based solder lowers thermal shock resistance of joined productsobtained by use of the solder. Wettability of Sn-9Zn eutectic alloy canbe improved by, for example, employment of flux. However, the problem ofthe chip-standing phenomenon arises upon mounting of electronic parts.

Conventional Sn—Pb based solder members have high stability, by virtueof a considerably small difference in oxidation potential between Sn andPb. In contrast, in the aforementioned Pb-free solder members, Snexhibits a considerable difference in oxidation potential from Ag, Cu orZn, thereby promoting oxidation of solder metal, resulting in a decreasein solderability. In particular, soldering by use of Sn—Zn based solderin the air has been difficult, since Zn has an oxidation potential lowerthan that of Pb, resulting in severe oxidation. In order to performsoldering in the air, a large amount of strong active agent must beincorporated into flux. In this case, solder metal readily reacts withthe flux, and this reaction deteriorate the stability of the solderpaste employed in the soldering.

In view of the foregoing, one object of the present invention is toprovide solder metal and solder paste exhibiting improved wettabilityand attaining high thermal shock resistance of joined portions whensoldering fine-pitch or large-scaled electronic parts onto a substrate.

Another object of the invention is to provide Sn—Zn—Bi solder metal andsolder paste capable of suppressing a decrease in thermal shockresistance of portions to be joined.

Still another object of the invention is to provide soldering flux thatcan suppress reaction with solder metal to obtain highly stable solderpaste.

DISCLOSURE OF THE INVENTION

The present inventor provides solder metal consisting essentially of 8.8to 5.0 mass % of Zn, 0.05 to 0 mass % of Bi and the balance of Sn andunavoidable impurities.

The invention further provides solder metal consisting essentially of9.0 to 5.0 mass % of Zn, 2.0 to 0.05 mass % of Bi and the balance of Snand unavoidable impurities.

The invention further provides solder metal consisting essentially of8.5 to 6.5 mass % of Zn, 1.5 to 0.1 mass % of Bi and the balance of Snand unavoidable impurities.

The invention further provides solder metal consisting essentially of8.3 to 7.5 mass % of Zn, 1.2 to 0.5 mass % of Bi and the balance of Snand unavoidable impurities.

Each solder metal comprises solder powder containing solder particleshaving a particle size of 20 μm or less in an amount controlled to 30%or less as determined in a number-basis particle-size distributionprofile.

The present invention also provides soldering flux comprising ahydrocarbon compound having at least one primary, secondary or tertiaryCH bond and a halogen-containing and hydrogen-donating compound.

The halogen-containing and hydrogen-donating compound can be an organichalogenated compound.

The hydrocarbon compound having at least one primary, secondary ortertiary CH bond can be one species selected from the group consistingof tetralin, tetraisobutylene, octahydroanthracene,1-α-naphthyl-1-n-butyl-hexadecene,9,10-dihydro-9,10-diisobutylanthracene, n-octadecylbenzene,β-n-octadecyltetralin and polyisobutylene.

The halogen-containing and hydrogen-donating compound is contained in anamount falling within a range of 0.02 to 20 mass % in terms of chlorinebased on a total amount of the soldering flux.

The hydrocarbon compound having at least one primary, secondary ortertiary CH bond is contained in an amount falling within a range of0.01 to 20 mass %, based on a total amount of the soldering flux.

The present invention also provides solder paste comprising the soldermetal and the soldering flux.

The solder metal is contained in an amount falling within a range of 86to 92 mass % and the soldering flux is contained in an amount fallingwithin a range of 14 to 8 mass %, respectively, based on a total amountof the solder paste.

The present invention also provides a method for producing a printedcircuit board, comprising the steps of applying the solder paste onto acircuit board, placing electronic parts on portions to which the solderpaste has been applied and heating the circuit board so as to mount theelectronic parts on the circuit board.

The invention also provides liquid flux for use in flow solderingcomprising the soldering flux that is diluted with a solvent.

The invention also provides rosin-containing solder thread comprisingthe solder metal and the soldering flux.

In the solder metal or solder paste according to the present invention,by reducing the Zn content to approximately 7% as described above, it ispossible to prevent oxidation of Sn—Zn solder metal and, by usingspecific flux in combination with the Sn—Zn solder metal, it is possibleto improve wettablity of the solder metal to a great extent and enhancethe property of mounting electronic parts on a circuit board and thethermal shock resistance of the mounted portions.

In the Sn—Zn—Bi based solder metal, by setting the Bi content atapproximately 1%, it is possible to improve the thermal shock resistanceof the mounted portions to a great extent and, by using as an activeagent a combination of a hydrocarbon compound having at least oneprimary, secondary or tertiary CH bond and a halogen-containing andhydrogen-donating compound, it is possible to obtain highly stablesolder paste improved in the solderability.

BEST MODES FOR CARRYING OUT THE INVENTION

The present inventors have conducted extensive studies on the amount ofBi to be added to Sn—Zn based solder that has attracted attention aspromising Pb-free solder and, as a result, have found that thermal shockresistance can be remarkably improved by reducing the amount of Bi addedto the solder to approximately 1% and that wettability can be remarkablyimproved by use of specific flux. Consequently, it is made possible toprovide a solder material that attains high joining reliability.

The present inventors have also found that reducing the Zn content toapproximately 7% enables oxidation of Sn—Zn binary solder alloy to beprevented, wettablity of the binary solder alloy to be improved and chipstanding phenomenon to be suppressed and that wettability can beremarkably improved by use of specific flux in combination. Consequentlyit is made possible to provide a solder material that attains excellentproperty of mounting electronic parts onto a circuit board.

The inventors have also found that, by use of an active agent containinga halogen-containing and hydrogen-donating compound and a hydrocarboncompound having at least one primary, secondary or tertiary CH bond,soldering in air by use of Pb-free solder, which conventionally has beendifficult to perform, can be effectively performed. The presentinvention has been accomplished on the basis of these findings.

The present invention provides solder metal consisting essentially of8.8 to 5.0 mass % of Zn, 0.05 to 0 mass % of Bi and the balance of Snand unavoidable impurities, solder metal consisting essentially of 9.0to 5.0 mass % of Zn, 2.0 to 0.05 mass % of Bi and the balance of Sn andunavoidable impurities, solder metal consisting essentially of 8.5 to6.5 mass % of Zn, 1.5 to 0.1 mass % of Bi and the balance of Sn andunavoidable impurities, and solder metal consisting essentially of 8.3to 7.5 mass % of Zn, 1.2 to 0.5 mass % of Bi and the balance of Sn andunavoidable impurities. Use of the solder metal having theaforementioned ranges of the components can attain high wettability,high thermal shock resistance of joined portions and high solderabilitywhen soldering electronic parts onto a circuit board.

The term “unavoidable impurities” refers to elements that are inevitablyintermingled during production of solder metal. The unavoidableimpurities may alternatively be defined as elements that do not greatlyaffect the characteristics of solder metal even when these elements areintentionally added thereto. Generally, the amount of each of theunavoidable impurities mingled into solder metal is 1 mass % or less.Examples of the unavoidable impurities include non-metallic elements,semi-metallic elements, carbon, oxygen, nitrogen and transition metals.Of these, unavoidable elements, such as Pb, Ag, Sb, Cu, Fe, Al, As, Cd,etc., are readily migrated to solder metal. However, since interminglingof these elements does not adversely affect the characteristics of thesolder metal of the present invention, they can be defined asunavoidable impurities.

The reason for controlling the composition of the solder metal so as tofall within the aforementioned ranges is as follows. In the case ofSn—Zn binary alloy, when Zn is contained in an amount higher than 8.8mass %, the solder metal undergoes severe oxidation, and wettability isconsiderably deteriorated. In this case, when a parts-placed circuitboard is reflowed, a chip standing phenomenon occurs. When the Zncontent is less than 5.0 mass %, the melting temperature (liquidus)thereof is elevated to higher than 215° C., resulting in considerableshortening of the service life of mounted electronic parts attributableto their heat resistance. By controlling the Bi content to 0.05 mass %or less or to 0, reliability of joined portions and wettability can beenhanced.

In the case of Sn—Zn—Bi ternary alloy, when Zn is contained in an amountgreater than 9.0 mass %, the solder metal undergoes severe oxidation,and wettability is deteriorated considerably. When the Zn content isless than 5.0 mass %, the melting temperature thereof is elevated to215° C. or higher, resulting in considerable shortening of the servicelife of electronic parts attributable to their heat resistance. Thus,the Zn content is preferably controlled to 8.5 to 6.5 mass %, morepreferably 8.3 to 7.5 mass %, from the viewpoint of prevention ofoxidation and enhancement in wettability of the solder metal. When theBi content is in excess of 2.0 mass %, thermal shock resistance ofelectronic-part-joined portions is deteriorated considerably, whereaswhen the Bi content is less than 0.05 mass %, wettability isdeteriorated considerably. In order to attain required thermal shockresistance and high wettability, the Bi content is preferably controlledto 1.5 to 0.1 mass %, more preferably 1.2 to 0.5 mass %.

According to the present invention, a kneaded product of a flux and asolder powder formed from any one of the aforementioned solder metals ispreferably employed as a solder paste. Preferably, in the solder paste,the solder powder content falls within a range of 86 to 92 mass %, andthe flux content falls within a range of 8 to 14 mass %, based on thetotal amount of the solder paste.

The atomizing method can be raised as a typical method for producingsolder powder from solder metal. More specifically, atomizing either bymeans of a disk-type atomizer or by means of spraying may be employed.The atomizing is preferably performed in an inert gas atmosphere, suchas an atmosphere of nitrogen, argon or helium, in order to preventoxidation of the solder powder. Needless to say, atomizing may beperformed under vacuum. Generally, the thus-produced solder powder isclassified by means of a classifier. The particle size range varies inaccordance with the pitch of electronic parts to be mounted. Whenfine-pitch parts, such as 0.3-mm-pitch parts, 0603 chip parts and 0.5-mmCSPs, are to be mounted, solder powder classified to a particle sizerange of 38 to 22 μm, typically to 45 to 22 μm, is used. Although theclassifier may be a vibration classifier or an airflow classifier,classification is preferably performed in an inert gas (e.g., nitrogen)flow so as to prevent oxidation. In addition, wet classification by useof a solvent may also be employed.

The particle size of solder powder is classified into some range groupsas defined by the Japanese Industrial Standards (JIS); e.g., 63-22 μm,45-22 μm and 38-22 μm. The particle size of solder powder is typicallydetermined through a method defined in JIS employing standard sieves anda balance. However, when the method is employed, solder microparticleswhich often adhere onto the surfaces of solder powder particles by, forexample, electrostaticity cannot be sufficiently removed, and the totalamount of the solder microparticles determined becomes smaller than theactual total amount of the microparticles contained in the solderpowder. When the solder powder that has undergone classificationperformed during particle size determination (e.g., in accordance withJIS) is observed under a microscope, a large number of soldermicroparticles are found to adhere onto a large solder particle. Whenthe amount of such microparticles contained in solder powder increases,the solder powder is readily oxidized, thereby deteriorating storagestability and reflow characteristics of the solder paste.

The present inventors have found that solder powder of excellentcharacteristics can be obtained by controlling the particle size of thepowder on the basis of a number-basis particle-size distribution profileof the solder particles, in addition to the method for determining theparticle-size distribution defined in JIS.

The microparticle content in solder powder can also be determined byimage analysis using a microscope or the electrozone method using aCoulter counter. The principle of a Coulter counter is described in“Powder Industry Handbook” edited by Powder Industry Association, 2nded., pp. 19-20). Specifically, a dispersion of powder is caused to passthrough orifices provided in a separator, and a change in electricalresistance is detected by a pair of electrodes disposed on therespective sides of the separator. The particle-size distribution of thepowder is determined on the basis of the change in electricalresistance, which depends of the particle size. According to thismethod, proportions in number of powder particles can be determined withhigh reproducibility. Employing the method for obtaining a particle-sizedistribution profile of solder powder enables quantitation ofmicroparticles adhering onto the surfaces of solder particles, whichmicroparticles tend to be released from the solder powder upondispersing the powder in liquid and have not been detected on the basisof a mass-basis or volume-basis particle-size distribution profileobtained through a conventional method employing sieves.

The lower limit of particle sizes that can be detected by image analysisusing a microscope or by the Coulter counter method is about 1 μm. Thus,through employment of any of the above methods, the amounts ofmicroparticles having a particle size of 1 μm or less are difficult todetermine. However, since solder powder produced through a typicalatomizing method contains substantially no microparticles having aparticle size of 1 μm or less, the aforementioned number-basisparticle-size distribution profile of the solder microparticles can beperformed for particles having a particle size of at least 1 μm.

According to the present invention, the amount of solder particleshaving a particle size of 20 μm or less contained in the solder powderis controlled to 30% or less, preferably 20% or less, as determined in anumber-basis particle-size distribution profile. When the amount ofsolder particles having a particle size of 20 μm or less is in excess ofthe above upper limit, the surface area per unit mass increases, therebyreadily causing oxidation of the powder. Thus, when solder paste isproduced by use of such solder powder, melting performance during areflow process of the solder powder contained in the solder paste isadversely affected. In addition, such powder readily reacts with flux,thereby shortening the shelf life of the solder paste and deterioratingtackiness of the solder paste.

Several methods for reducing the amount of microparticles contained insolder powder can be employed; e.g., presetting the particle sizeclassification range of the solder powder upon classification to begreater than the target particle size range; repeating classification byuse of sieves until the level of micropowders contained in the solderpowder is lowered below the target level; retarding the feeding rate ofthe powder so as to facilitate removal of microparticles; and performingwet classification by use of a solvent other than water.

The solder powder employed in the present invention contains solderparticles having a particle size not greater than the size of the sievewhich determines the upper limit of particle size during classificationof at least 90%, preferably at least 95%, as determined in a mass-basisparticle-size distribution profile.

Atomic oxygen content of the solder powder employed in the presentinvention is preferably as low as possible. Specifically, the oxygenatom content is preferably controlled to 500 ppm or less, morepreferably 300 ppm or less, so as to enhance storage stability andreflow characteristics of the solder paste. In order to effectivelylower the oxygen atom content in the solder powder, an atomizing stepfor producing solder powder is performed in an oxidation-resistiveatmosphere, and the thus produced solder powder is handled in anoxidation-resistive atmosphere. More specifically, the aforementionedsteps are preferably preformed in a vacuum or in an atmospherecontaining inert gas, such as nitrogen gas, argon gas or helium gas.

Generally, soldering flux, solder paste, solder thread or liquid fluxfor flow soldering contains an active agent, a resin component formed ofsynthetic resin or rosin, a solvent, a thixotropic agent and optionaladditives, such as a pH-regulator, an anticorrosion agent, ananti-oxidant and an organic acid component.

The soldering flux of the present invention is characterized bycontaining, as active agents, a hydrocarbon compound having at least oneprimary, secondary or tertiary CH bond, and a halogen-containing andhydrogen-donating compound.

The halogen-containing and hydrogen-donating compound is ahalogen-containing compound that serves as a reducing agent in thesoldering flux. Among such compounds, particularly preferred is acompound that thermally decomposes at the temperature of application ofthe soldering flux to thereby generate a halogen.

Examples of the above halogen-containing compound include an organichalogenated compound that thermally decomposes at the temperature ofapplication of the soldering flux to thereby generate hydrogen halide.

Conventionally, a variety of ionic active agents have been employed.However, such ionic active agents, which also exert activity at roomtemperature, adversely affect storage stability of Pb-free solder thatis particularly oxidative. Therefore, an organic bromine compound thatexerts activity through decomposition upon a reflow process ispreferably added to Pb-free solder. JP-B SHO 56-32079, JP-B HEI 4-59079,JP-A HEI 3-106594, JP-A HEI 8-155676 and other publications discloseaddition of an organic halogenated compound to soldering flux.

Examples of organic halogenated compounds which are preferably used inthe present invention include 1-bromo-2-butanol, 1-bromo-2-propanol,3-bromo-1-propanol, 3-bromo-1,2-propanediol, 1,4-dibromo-2-butanol,1,3-dibromo-2-propanol, 2,3-dibromo-1-propanol,1,4-dibromo-2,3-butanediol, 2,3-dibromo-2-butene-1,4-diol,1-bromo-3-methyl-1-butene, 1,4-dibromobutene, 1-bromo-1-propene,2,3-dibromopropene, ethyl bromoacetate, ethyl α-bromocaprylate, ethylα-bromopropionate, ethyl β-bromopropionate, ethyl α-bromoacetate,2,3-dibromosuccinic acid, 2-bromosuccinic acid, 2,2-dibromoadipic acid,2,4-dibromoacetophenone, 1,1-dibromotetrachloroethane,1,2-dibromo-1-phenylethane, 1,2-dibromostyrene, 4-stearoyloxybenzylbromide, 4-stearyloxybenzyl bromide, 4-stearylbenzyl bromide,4-bromomethylbenzyl stearate, 4-stearoylaminobenzyl bromide,2,4-bisbromomethylbenzyl stearate, 4-palmitoyloxybenzyl bromide,4-myristoyloxybenzyl bromide, 4-lauroyloxybenzyl bromide,4-undecanoyloxybenzyl bromide, 9,10,12,13,15,16-hexabromostearic acid,methyl 9,10,12,13,15,16-hexabromostearate, ethyl9,10,12,13,15,16-hexabromostearate, 9,10,12,13-tetrabromostearic acid,methyl 9,10,12,13-tetrabromostearate, ethyl9,10,12,13-tetrabromostearate, 9,10,12,13,15,16-hexabromostearylalcohol, 9,10,12,13-tetrabromostearyl alcohol,1,2,5,6,9,10-hexabromocyclododecane, bis(2,3-dibromopropyl) succinate,bis(2,3-dibromopropyl) o-phthalate, bis(2,3-dibromopropyl) p-phthalate,bis(2,3-dibromopropyl)-o-phthalamide,bis(2,3-dibromopropyl)-p-phthalamide, tris(2,3-dibromopropyl)trimellitate, tris(2,3-dibromopropyl)-trimellitamide,tetra(2,3-dibromopropyl) pyromellitate,tetra(2,3-dibromopropyl)-pyromellitamide,bis(2,3-dibromopropyl)-glycerol,trimethylolpropanebis(2,3-dibromopropyl) ether,bis(2,3-dibromopropyl)-tartamide,N,N′-bis(2,3-dibromopropyl)-succinamide,N,N,N′,N′-tetra(2,3-dibromopropyl)-succinamide,N,N′-bis(2,3-dibromopropyl)-urea,N,N,N′,N′-tetra(2,3-dibromopropyl)-urea,2,2-bis[4-(2,3-dibromopropyl)-3,5-dibromophenyl]propane,α,α,α-tribromomethylsulfone, α,β-dibromoethylbenzene andtris(2,3-dibromopropyl) isocyanurate. Of these,9,10,12,13,15,16-hexabromostearic acid, methyl9,10,12,13,15,16-hexabromostearate, ethyl9,10,12,13,15,16-hexabromostearate and tris(2,3-dibromopropyl)isocyanurate are particularly preferably used. Although the reason whythese organic halogenated compounds are preferably used with solderingflux has not been elucidated, a conceivable reason is that thetemperature at which these compounds thermally decompose to therebygenerate hydrogen halide is approximately equal to the preheatingtemperature.

Alternatively, corresponding organic halogenated compounds containingchlorine or iodine instead of bromine may be used. These compounds maybe used singly or in combination of two or more species.

The organic halogenated compound is added to soldering flux in an amountfalling within a range of 0.02 to 20 mass %, preferably 0.1 to 10 mass%, in terms of chlorine-based on the total amount of the soldering flux.When the amount is less than 0.02 mass %, solderability during a reflowprocess is attained insufficiently, whereas when the amount is in excessof 20 mass %, cost disadvantageously increases and the relative amountsof other flux components are reduced, thereby failing to satisfactorilyattain other functions required for flux.

The amount in terms of chlorine is calculated on the basis of the ratioof the molecular weight of the organic chlorine-containing compound tothat of the organic bromine-containing compound.

An ionic bromine-containing active agent exerts its effect throughaddition thereof in a small amount and does not affect the stability ofpaste containing the agent. In order to reduce the amount of anexpensive organic halogenated compound, the ionic bromine-containingactive agent may be used in combination with the organic halogenatedcompound.

Examples of other ionic active agents that are preferably added includeamine hydrohalides, such as isopropylamine hydrobromide, butylaminehydrochloride and cyclohexylamine hydrobromide, and1,3-diphenylguanidine hydrobromide.

Hydrochlorides and hydroiodides may be used instead of hydrobrimides.These hydrohalides may be used singly or in combination of two or morespecies. The amount of hydrohalide falls within a range of 0.0005 to 2mass %, preferably 0.01 to 1 mass %, based on the total amount of flux.When the amount is less than 0.0005 mass %, activity thereof is poor,whereas when the amount is in excess of 2 mass %, the hydrohalide reactswith an active agent contained in the flux, thereby deterioratingstability of solder paste.

According to the present invention, a hydrocarbon compound having atleast one primary, secondary or tertiary CH bond is added. Preferably,the hydrocarbon compound is readily oxidized by oxygen at about 100 toabout 150° C. Examples of such compounds include tetralin,tetraisobutylene, octahydroanthracene, 1-α-naphthyl-1-n-butylhexadecene,9,10-dihydro-9,10-diisobutylanthracene, n-octadecylbenzene,β-n-octadecyltetralin, polyisobutylene, α-phenyl-2-tetralylbutane,9,10-diisobutylperhydroanthracene and 5-isobutylacenaphthene. Thehydrocarbon compound is added in an amount falling within a range of0.01 to 20 mass %, preferably 0.5 to 5 mass %, based on the total amountof the flux.

Although the oxidation mechanism has not been elucidated in detail, thefollowing mechanism is conceived. The hydrocarbon compound having atleast one primary, secondary or tertiary CH bond is oxidized by oxygenat a typical preheating temperature of about 100 to about 150° C., andthe halogen-containing and hydrogen-donating compound catalyzes theabove oxidation.

When conventional solder paste is preheated at about 150° C., solderpowder is considerably oxidized. However, since the solder paste of thepresent invention contains a hydrocarbon compound having at least oneprimary, secondary or tertiary CH bond, the hydrocarbon compound reactswith oxygen at preheating temperature to thereby incorporate oxygen intothe compound, and oxidation of the solder powder at preheatingtemperature is prevented. In addition, hydrogen halide reduces oxidizedportions of the solder surface, thereby possibly enhancingsolderability.

The soldering flux of the present invention may further contain asolvent, a resin component, a thixotropic agent and optional additivesthat include an anticorrosion agent, an anti-oxidant and a pH-regulator.

The soldering flux of the present invention may employ solvents used inconventional fluxes and solder pastes, specifically, alcohols, ethers,esters, middle or higher glycols (alcohols) and aromatic solvents. Thesesolvents may be used singly or in combination of two or more species.Specific examples include benzyl alcohol, butanol, ethyl cellosolve,butyl cellosolve, butyl carbitol, propylene diglycol monobutyl ether,diethylene glycol monophenyl ether, diethylene glycol hexyl ether,propylene glycol monophenyl ether, diethylene glycol monohexyl ether,diethylene glycol mono-2-ethylhexyl ether, dioctyl phthalate, xylene,2-methyl-1,3-hexanediol, 2-ethyl-1,3-hexanediol and mixtures thereof.

A known resin that has conventionally been blended into flux may beblended into the soldering flux of the present invention. Examples ofsuch resins include natural rosin, disproportionated rosin, polymerizedrosin, hydrogenated rosin, modified rosin, rosin derivatives, such asrosin esters, and synthetic resins, such as polyester, polyurethane andacrylic resin.

A thixotropic agent to be added for the improvement of printability maybe an inorganic substance, such as fine silica particles or kaolinparticles, or an organic substance, such as hydrogenated castor oil oran amide compound.

Examples of the pH-regulator that may be added to the soldering flux ofthe present invention include amine compounds, such as alkanolamines,aliphatic primary to tertiary amines, aliphatic unsaturated amines,alicyclic amines and aromatic amines. These amines may be used singly orin combination of two or more species.

The pH of the soldering flux of the present invention preferably fallswithin a range of 4 to 9, more preferably 6 to 8, from the viewpoint ofsuppressing reaction of solder powder with the flux. According to thepresent invention, the pH-regulator is preferably added in an amountfalling within a range of 0.05 to 20 mass % based on the total amount ofthe soldering flux. When the amount is less than 0.05 mass %, effect ofthe pH-regulator is poor, whereas when the amount is in excess of 20mass %, the solder paste tends to become hygroscopic, which is notpreferred.

Specific examples of the above amine compounds include ethanolamine,butylamine, aminopropanol, polyoxyethylene oleylamine, polyoxyethylenelaurylamine, polyoxyethylene stearylamine, diethylamine, triethylamine,methoxypropylamine, dimethylaminopropylamine, dibutylaminopropylamine,ethylhexylamine, ethoxypropylamine, ethylhexyloxypropylamine,bispropylamine, isopropylamine, diisopropylamine, piperidine and2,6-dimethylpiperidine.

To the soldering flux of the present invention, any azole may be addedas an anticorrosion agent. Examples of such azoles includebenzotriazole, benzimidazole and tolyltriazole. Such an anticorrosionagent is added preferably in an amount falling within a range of 0.05 to20 mass % based on the total amount of flux.

The soldering flux of the present invention preferably contains ananti-oxidant. Anti-oxidants that serve as typical reducing agents ofsynthetic resin or similar material and can be dissolved in a solventare used as the above anti-oxidant. Examples include phenolic compounds,phosphate compounds, sulfur-containing compounds, tocopherol and itsderivatives, and ascorbic acid and its derivatives. These reducingagents may be used singly or in combination. The reducing agent is addedin an amount falling within a range of 0.0005 to 20 mass %, preferably0.01 to 10 mass %, based on the total amount of the flux.

Although the mechanism of action of the added anti-oxidant has not beenelucidated in detail, the anti-oxidant is conceived to interact withoxygen dissolved in solder paste or contained in air, thereby preventingoxidation of solder metal. Furthermore, it is also conceived that theanti-oxidant also serves as an acceptor for capturing halogen speciesreleased from a halogen-containing component, thereby effectivelyinhibiting reaction of the released halogen species with solder metal.

Examples of the organic acid component include conventionally knownacids, such as succinic acid, phthalic acid, stearic acid and sebacicacid. Derivatives of such acids, which are compounds that generate anorganic acid when the derivatives reach the reflow temperature, arepreferably used. Organic acid esters that decompose to form thecorresponding acids exhibit poor decomposability at the reflowtemperature when used singly. Thus, a small amount of an esterdecomposition catalyst is effectively added in order to promotedecomposition. No particular limitation is imposed on the esterdecomposition catalyst, so long as the catalyst promotes generation ofacid through decomposition of a decomposable organic acid ester at thereflow temperature. Among such catalysts, organic bases and hydrohalidesalts are particularly effective.

Examples of organic acid esters include various aliphatic carboxylicacid esters, aromatic carboxylic acid esters, aliphatic sulfonic acidesters and aromatic sulfonic acid esters. Specific examples includen-propyl p-toluenesulfonate, isopropyl p-toluenesulfonate, isobutylp-toluenesulfonate, n-butyl p-toluenesulfonate, n-propylbenzenesulfonate, isopropyl benzenesulfonate, isobutyl benzenesulfonate,n-propyl salicylate, isopropyl salicylate, isobutyl salicylate, n-butylsalicylate, isopropyl 4-nitrobenzoate, t-butyl 4-nitrobenzoate, t-butylmethacrylate, t-butyl acrylate, t-butyl malonate and t-butylbromoacetate. The amount of the organic acid ester to be added is in therange of 0.01 to 20 mass %, preferably 0.05 to 5 mass %, based on thetotal amount of flux.

In the present invention, solder powder made from the solder metal ispreferably kneaded with flux to form solder paste. In this case, theratios of the solder powder and flux are preferably in the ranges of 86to 92 mass % and of 14 to 8 mass %, respectively, based on the totalamount of the solder paste.

The solder paste of the present invention is advantageously used inproducing a joined product having electronic parts joined to asubstrate, such as a circuit board. In a method of using the flux andsolder paste of the present invention and a method of producing joinedproducts, the solder paste is applied to portions of a circuit boardintended for soldering by a printing method or similar means, electronicparts are placed on the portions, and the circuit board is then heatedto melt the solder particles. The melted solder particles are allowed tosolidify to enable the electronic parts to be joined to the circuitboard.

A typical method for joining a substrate and electronic parts (i.e., amounting method) is a surface mounting technology (SMT). This mountingmethod involves applying solder paste to a desired portion on asubstrate, such as a wiring board, through printing, subsequentlyplacing electronic parts, such as chip parts and QFPs, on the appliedsolder paste, and soldering the entirety by means of a reflow heatsource. Examples of such a reflow heat source include a hot air chamber,an infrared radiation chamber, a vapor phase condensation solderingapparatus and a light-beam soldering apparatus.

The reflow process of the present invention is performed preferably intwo steps, i.e., the step of preheating and the step reflow. Regardingconditions, the preheating temperature is 130 to 180° C., preferably 130to 150° C. The preheating time is 60 to 120 seconds, preferably 60 to 90seconds. The reflow temperature is 210 to 230° C., preferably 210 to220° C. The reflow time is 30 to 60 seconds, preferably 30 to 40seconds.

When the solder paste of the present invention is used, theaforementioned reflow process can be carried out both in a nitrogenatmosphere and in air. When the nitrogen atmosphere is chosen, theoxygen concentration of the atmosphere is controlled to 5 vol % or less,preferably 0.5 vol % or less, to thereby enhance wettability of solderto a substrate, such as a wiring board, as compared with a reflowprocess in air. In addition, generation of solder balls is suppressed tothereby attain smooth treatment.

Subsequently, the reflowed substrate is cooled to complete surfacemounting. In this mounting method, joining may be effected on both sidesof a substrate, such as a printed wiring board (onto which electronicparts are to be mounted), for producing electronic-parts-mountedproducts. No particular limitation is imposed on the electronic parts towhich the solder paste of the present invention can be applied. Examplesof the electronic parts include LSIs, resistors, capacitors,transducers, inductors, filters, oscillators and vibrators.

Alternatively, mounting is carried out by use of the solder paste of thepresent invention through the SMT (surface mounting technology) on acircuit substrate which is prepared in the following manner: forming inadvance adhesive coating film exclusively on a predetermined surfaceportion of a substrate (e.g., metallic wiring of a printed wiring board)by means of chemical reaction; depositing solder powder on the adhesivecoating film; applying flux thereon and reflowing by heating to themelting temperature of the solder, to form solder bumps on the circuitsubstrate (JP-A HEI 7-7244). In this case, excellent solderability canbe attained.

As compared with conventional solder metal and solder paste, the soldermetal or solder paste of the present invention exhibits excellentcharacteristics, such as reflow characteristics, solderability,wettability to a metal to be joined and printability, and suppressesgeneration of solder balls during a reflow process. Particularly, thesolder metal or solder paste of the present invention exhibitsremarkably enhanced wettability to electronic parts and circuitsubstrates to be joined and remarkably enhances thermal shock resistanceof joined products. Furthermore, the Pb-free solder alloy of the presentinvention, generating less environmental-pollutant waste, is suited forattaining fine-pitch mounting of electronic parts (e.g., producingfine-pitch electronic-parts-mounted circuit boards) and is adapted to avariety of electronic parts. Thus, the present invention can provide awiring board of a long service life of the electronic parts mountedthereon.

EXAMPLES

The present invention will next be described in more detail by way ofexamples, which should not be construed as limiting the inventionthereto.

<Test Methods>

(1) Number-Basis Distribution Profile of Solder Microparticles:

A Coulter counter (Multi-sizer model II, product of Coulter Electronics)was employed. A solder powder (1 g) was dispersed in a 1% NaClelectrolyte (100 mL). The thus yielded dispersion was placed in adetector equipped with a detection tube (orifice size: 400 μm) andanalyzed to thereby obtain a particle size distribution profile. Theproportion of the powder (particle size of 1 μm or more) in the particlesize distribution profile was obtained.

(2) Oxygen Content:

The oxygen content was determined by use of an oxygen analyzer (productof RECO) through the IR absorption method.

(3) Water Content:

Solder paste was placed in a water vaporization apparatus (ADP-351,product of Kyoto Denshi Kogyo) and heated to 150° C. so as to vaporize.The formed gas was fed to a Karl Fischer water content meter (MKC-210,product of Kyoto Denshi Kogyo) by the mediation of nitrogen serving as acarrier gas, whereafter the water content of the gas was determined.

(4) pH:

Flux (4 g) was dissolved in a mixture of toluene (50 mL), isopropylalcohol (49.5 mL) and water (0.5 mL). The thus yielded solution wassubjected to pH measurement by means of a pH meter. When the sample issolder paste, the solder paste was weighed in an amount corresponding to4 g of flux, and pH was measured in a similar manner.

(5) Solder Ball Test 1:

A solder ball test was performed in accordance with JIS Z-3284. Eachsolder paste was applied to an alumina test plate through printing byuse of a metallic mask (thickness: 0.2 mm) having four holes (diameter:6.5 mm) to thereby prepare disk-form test samples. Each of these sampleswas dried at 150° C. for one minute and then heated to 235° C. tothereby melt the solder. Within five seconds after melting of thesolder, the sample was removed in a horizontal orientation and allowedto stand in a horizontal orientation until the solder on the sample wassolidified. Subsequently, the general appearance of the thus-solidifiedsolder was observed under a magnifying lens (×20), and generation ofsolder microparticles around solder particles was observed under amagnifying lens (×50). Each sample was evaluated in terms of solder ballgeneration on the basis of the standards described in JIS Z-3284.Samples rated 3 or less were evaluated to be test-not-passed samples(denoted by x in Table 1), and those rated higher than 3 were denoted by⊚ {circle around (∘)}, ◯ and Δ in the order of excellence of state.

(6) Wettability:

Wettability was determined in accordance with JIS Z-3284. Each solderpaste was applied to a copper test plate through printing by use of ametallic mask (thickness: 0.2 mm) having four holes (diameter: 6.5 mm)to thereby prepare disk-form test samples. Each of these samples wasdried at 150° C. for one minute and then heated to 235° C. to therebymelt the solder. Within five seconds after melting of solder, the samplewas removed in a horizontal orientation and allowed to stand in ahorizontal orientation until the solder on the sample was solidified.Subsequently, each sample was evaluated in terms of wettability on thebasis of the standards described in JIS Z-3284. Samples rated 3 or lesswere evaluated to be test-not-passed samples (denoted by x in Table 1),and those rated higher than 3 were denoted by ⊚ {circle around (∘)}, ◯and Δ in the order of excellence of wettability.

(7) Shock Resistance Test:

Each test substrate (thickness: 1.6 mm, 100 mm×100 mm) was subjected toa shock resistance test. L-QFP (100 pins mounted on copper lead viaSn-10Pb plating) was employed as an electronic part. Prior to the test,a copper pattern formed on the test substrate was etched in a depth of 1to 2 μm with a soft-etching liquid (MECBRITE CB-801, product of MEC Co.,Ltd.) and then treated with a heat-resistant pre-flux (MEC Pre-fluxR-4030, product of MEC Co., Ltd.). Each solder paste was applied to thetest substrate through printing by use of a 150-μ metallic mask, and theL-QFP (plated with Sn—Pb) was placed on the solder. The resultant testsubstrate was subjected to a reflow step (pre-heating at 160° C. for 90seconds, peak temperature of 220° C. and maintenance at 200° C. orhigher for 30 seconds). Each test substrate was subjected to the reflowstep twice, in view that in actual situations, double-side reflowing isperformed. On one side (side on which no parts were mounted) of thethus-prepared test piece for thermal shock test, wire (φ 1.5 mm) wasdisposed in the center, with both ends being secured by means ofheat-resistant Kapton tape such that strain load is forcedly applied.The thus-modified test substrate was placed in a thermal shock tester,and joining strength was determined under cooling-heating (−40° C. for30 minutes, +125° C. for 30 minutes) cycles (determined at an initialstage and after 100 cycles, 300 cycles, and 500 cycles). The joiningstrength was determined in accordance with the 450 peel strength method(20 pins/unit) at a peeling speed of 20 mm/min. Determination of thestrength and observation of the peeled surface were performed by meansof a stereo-microscope (magnification: ×40). In the observation results,the case in which at least one pin was peeled was evaluated astest-not-passed (n Table 1, x denotes test-not-passed, and ◯ denotestest-passed).

(8) Chip Standing Test:

Each test substrate (thickness: 1.6 mm, 100 mm×100 mm) was subjected toa chip standing test. Parts “1608” (1.6 mm×0.8 mm, plated with Sn-10Pb)were employed as electronic parts (chips). Prior to the test, a copperpattern formed on the test substrate was etched in a depth of 1 to 2 μmwith a soft-etching liquid (MECBRITE CB-801, product of MEC Co., Ltd.)and then treated with a heat-resistant pre-flux (MEC Pre-flux R-4030,product of MEC Co., Ltd.). Each solder paste was applied on the testsubstrate through printing by use of a 150-μ metallic mask, and tenpieces of the chips (plated with Sn-10Pb) were placed on the printedsolder. The resultant test substrate was subjected to a reflow step(pre-heating at 160° C. for 90 seconds, peak temperature of 220° C., andmaintenance at 200° C. or higher for 30 seconds). The case in which nochip standing was observed was assigned rating ◯, the case in which onestanding pin was observed was assigned rating Δ, and the vase in whichtwo or more standing pins were observed was assigned rating X.

(9) Solder Ball Test 2:

Each solder paste was applied to an alumina substrate through printingby use of a printing mask (thickness: 0.2 mm) having five holes(diameter: 6.5 mm), to thereby prepare nine test samples. Eight of thenine test samples were placed in a thermostatic, hygrostatic chamber(30° C., 90%), and the eight samples were removed one by one atintervals of three hours.

Each of these samples was placed on a hot plate whose temperature hadbeen preset at 235° C. Five seconds after melting of solder, the samplewas removed and allowed to stand until the solder was cooled.

The general appearance of the thus-solidified solder was observed undera magnifying lens (×10), to thereby evaluate the solder on the basis ofthe aggregation state of solder particles as defined in Table 1 and FIG.1 of JIS Z-3284 (Appendix 11).

Specifically, evaluation was performed on the basis of the followingrating groups in terms of aggregation state of the melted solder powder:the case in which the melted solder forms one large ball, and no solderballs are observed around the large ball (rating 1); the case in whichthe melted solder forms one large ball, and three or more solder balls(diameter: ≦75 μm) are observed around the large ball (rating 2); thecase in which the melted solder forms one large ball, three or moresolder balls (diameter: ≦75 μm) are observed around the large ball, withthe solder balls forming a broken circle segment (rating 3); the case inwhich the melted solder forms one large ball, a large number of smallsolder balls (diameter: ≦75 μm) are observed around the large ball, withthe solder balls forming a broken circle (rating 4); and cases otherthan the above four cases (rating 5).

(10) Observation of Voids:

On each copper plate (60 mm×60 mm), 6 patterns (diameter 6 mm) wereformed through printing by use of a metal mask (thickness 150 μ). Eachof the samples was subjected to a reflow process under atmosphericconditions, and the resultant sample was cut by means of a cutter. Thecross-section of solder portions was observed under a microscope, tothereby investigate void generation. The number of voids of 10 μm insize or larger was counted in 6 patterns. When the average number perpattern was 2 or more, the sample was indicated as “test not passed.”

Examples 1 to 11 and Comparative Examples 1 to 8

The present invention will next be described in detail by way ofExamples and Comparative Examples. The modes for carrying out theinvention are not limited to Examples 1 to 11 and exemplified methodsfor producing an electronic-parts-joined product.

<Production of Flux and Solder Paste>

Each of flux samples was prepared by mixing the following components:polymerized rosin (17.5 mass %) and disproportionated rosin (27.5 mass%) serving as resin components; hydrogenated castor oil (6 mass %)serving as a thixotropic agent; n-propyl p-toluenesulfonate (0.5 mass %)serving as an organic acid ester; cyclohexylamine hydrobromide (0.08mass %) serving as an ester decomposition catalyst; hexabromostearicacid (3.5 mass % or 0) serving as an organic hydrogenated compound; amixture (1:1 by mass) of tocopherol and L-ascorbyl-2,6-dipalmitate (1.0mass %) serving as a reducing agent; triethylamine (2 mass %) serving asa pH-adjusting agent; tolyltriazole (1 mass %) serving as ananti-corrosive agent; and propylene glycol monophenyl ether (balance)serving as a solvent, to thereby attain the total amount of 100 mass %.

To each flux (11 mass %) was added each solder powder having a metalcomposition shown in Table 1 (particle size range: 20 to 45 μm,proportion in number of ≦20-μm particles in a particle size distributionprofile: 25% by number) (89 mass %), and the mixture was kneaded bymeans of a planetary mill, to thereby produce a solder paste (3 kg).Compositions of the solder pastes, compositions of the solder powdersemployed, and pH of fluxes are shown in Table 1. The solder powdersemployed were found to have an oxygen content of 300 ppm, and the solderpastes employed have a water content of 0.3 mass %.

<Production of Electronic-Parts-Joined Products>

Mounting of electronic parts by use of the solder paste of the presentinvention was performed through SMT. The solder paste having acomposition of Example 4 was applied to one sheet of a circuit boardthrough printing, and an LSI, a chip resistor, and a chip capacitor wereplaced on the solder paste. Then, the resultant assembly was heated by areflow heat source, to thereby effect soldering. The reflow heat sourcewas supplied from a hot air chamber.

The reflow process was performed under the following conditions:pre-heating temperature (130° C.), pre-heating time (80 seconds), reflowpeak temperature (220° C.), and reflow time at 200° C. or higher (50seconds).

The characteristics of the thus-produced printed wiring boards andsolder pastes used for producing the wiring boards were evaluatedthrough the aforementioned measurement methods. The results are shown inTable 1. As shown in solder ball test results, the solder pastes ofComparative Examples 1, 2, 3, and 8 generated a large amount ofundissolved solder powder.

Examples 12 to 17 and Comparative Example 9

<Production of Flux and Solder Paste>

The following raw materials were used: hydrogenated castor oil(thixotropic agent); disproportionated rosin or polymerized rosin (resincomponent); cyclohexylamine hydrobromide or 1,3-diphenylguanidinehydrobromide (ionic active agent); benzotriazole (anticorrosion agent);propylene diglycol monobutyl ether (solvent);9,10,12,13,15,16-hexabromostearic acid, methyl9,10,12,13,15,16-hexabromostearate, or tris(2,3-dibromopropyl)isocyanurate (organic halogenated compound); tripropylamine (amine); andn-propyl p-toluenesulfonate (organic acid component).

A powder (90 mass %) of solder, 91Sn/8Zn/1Bi, was added to eachsoldering flux (10 mass %), and the mixture was kneaded by means of aplanetary mill, to thereby produce a solder paste.

Table 2 shows compositions of solder pastes, and Table 3 shows resultsof evaluation. Furthermore, solder ball test 2 is used, and the resultsof the samples subjected to thermostatic and hygrostatic treatment for24 hours are shown in Table 3.

The solder paste samples of Examples 12 to 17 containing, as activeagents, a hydrocarbon compound having at least one primary, secondary,or tertiary CH bond, and a halogen-containing hydrogen-donating compoundexhibit remarkably excellent solderability, as compared with the solderpaste sample of Comparative Example 9, in the solder ball test and voidobservation.

TABLE 1 Solder Shock resistance test Solder composition Org. halogenatedball 100 300 500 Chip Zn Bi Sn compound pH test 1 Wettability Initialcycles cycles cycles standing Ex. 1 8.8 — bal Hexabromostearic acid 7.5⊚ ◯ ◯ ◯ ◯ ◯ Δ 2 7.0 — bal Hexabromostearic acid 6.9 ⊚ ⊚ ◯ ◯ ◯ ◯ ◯ 3 5.0— bal Hexabromostearic acid 7.2 Δ ◯ ◯ ◯ ◯ ◯ ◯ 4 7.0 0.05 balHexabromostearic acid 7.9 ⊚ ⊚ ◯ ◯ ◯ ◯ ◯ 5 8.0 2.0 bal Hexabromostearicacid 7.6 ⊚ ⊚ ◯ ◯ ◯ ◯ ◯ 6 8.0 1.0 bal Hexabromostearic acid 7.8 ⊚ ⊚ ◯ ◯ ◯◯ ◯ 7 9.0 2.0 bal Hexabromostearic acid 7 Δ ◯ ◯ ◯ ◯ ◯ ◯ 8 7.0 2.0 balHexabromostearic acid 7.1 ◯ ◯ ◯ ◯ ◯ ◯ ◯ 9 6.0 2.0 bal Hexabromostearicacid 7.3 Δ ◯ ◯ ◯ ◯ ◯ ◯ 10 5.0 2.0 bal Hexabromostearic acid 7.4 Δ Δ ◯ ◯◯ ◯ ◯ 11 8.0 1.0 bal Not added 7.9 ◯ ◯ ◯ ◯ ◯ ◯ ◯ Comp. 9.0 — balHexabromostearic acid 6.9 Unsol. X ◯ ◯ ◯ ◯ X Ex. 1 2 4.0 — balHexabromostearic acid 7.1 Unsol. X X X X X X 3 4.5 — balHexabromostearic acid 7.3 Unsol. X ◯ X X X Δ 4 8.0 6.0 balHexabromostearic acid 7.7 ⊚ ⊚ X X X X ◯ 5 8.0 3.0 bal Hexabromostearicacid 6.8 ⊚ ⊚ ◯ ◯ X X ◯ 6 9.0 2.5 bal Hexabromostearic acid 7 ◯ ◯ ◯ ◯ X X◯ 7 9.5 2.0 bal Hexabromostearic acid 7.3 Δ Δ ◯ ◯ ◯ X ◯ 8 4.5 0.05 balHexabromostearic acid 7.2 Unsol. X X X X X ◯

TABLE 2 Amount (mass %) Ex. 12 Ex. 13 Ex. 14 Ex. 15 Disproportionated26.5 26.5 26.5 26.5 rosin Polymerized rosin 17 17 17 17 Tripropylamine2.5 2.5 2.5 2.5 Hydrogenated 6 6 6 6 castor oil n-Propyl 0.05 0.05 0.050.05 p-toluenesulfonate Benzotriazole 1 1 1 1 Solvent PGBE PGBE PGBEPGBE Amount of Solvent 39.95 39.85 39.95 37.95 Ionic active agent —CHA-HBr 1,3-DPG-HBr CHA-HBr Amount of ionic — 0.1 0.1 0.1 active agentOrg. halogen compound 9,10,12,13,15,16-HBSA DBPI 9,10,12,13,15,16-HBSA9,10,12,13,15,16-HBSM Amount of org. 6 6 6 6 halogen compoundHydrocarbon compound Tetralin Tetralin Tetralin Tetralin Amount of 1 1 13 hydrocarbon compound Solder composition 91Sn/8Zn/1Bi 91Sn/8Zn/1Bi91Sn/8Zn/1Bi 91Sn/8Zn/1Bi Amount (mass %) Ex. 16 Ex. 17 Comp. Ex. 9Disproportionated 26.5 26.5 26.5 rosin Polymerized rosin 17 17 17Tripropylamine 2.5 2.5 2.5 Hydrogenated 6 6 6 castor oil n-Propyl 0.050.05 0.05 p-toluenesulfonate Benzotriazole 1 1 1 Solvent PGBE PGBE PGBEAmount of Solvent 39.95 39.95 40.95 Ionic active agent CHA-HBr CHA-HBrCHA-HBr Amount of ionic 0.1 0.1 0.1 active agent Org. halogen compound9,10,12,13,15,16-HBSA 9,10,12,13,15,16-HBSA 9,10,12,13,15,16-HBSA Amountof org. 6 6 6 halogen compound Hydrocarbon compound TetraisobutyleneOctahydroanthracene — Amount of 1 1 — hydrocarbon compound Soldercomposition 91Sn/8Zn/1Bi 91Sn/8Zn/1Bi 91Sn/8Zn/1Bi PGBE: Prolylenediglycol monobutyl ether, CHA-HBr: Cyclohexylamine hydrobromide,DPG-HBr: 1,3-Diphenylguanidine hydrobromide, HBSA: Hexabromostearicacid, DBPI: Tris(2,3-dibromopropyl)isocyanurate, HBSM: Methylhexabromostearate

TABLE 3 Ex. 12 Ex. 13 Ex. 14 Ex. 15 Ex. 16 Ex. 17 Comp. Ex. 9 Solderball test 2 1 1 1 1 1 1 3 Void observation Passed Passed Passed PassedPassed Passed Not passed

INDUSTRIAL APPLICABILITY

As described hereinabove, by use of the solder paste containing a soldermetal and a flux falling within the scope of the present invention, highwettability to electronic parts, high thermal shock resistance, andreduction of occurrence of chip standing can be attained. In addition,reaction between solder powder and flux is remarkably suppressed,thereby attaining remarkably excellent solderability.

As described hereinabove, solderability can be enhanced by employing, asan active agent for lead-free solder, the soldering flux of the presentinvention containing a halogen-containing hydrogen-donating compound,and a hydrocarbon compound having at least one primary, secondary, ortertiary CH bond. Particularly, solder balls and voids in bumps that aregenerated after reflowing can be reduced.

The solder paste of the present invention can provide circuit boards; amethod for soldering a circuit board; soldered circuit boards; a methodfor joining electronic parts; and joined products, suited for producingfine-pitch electronic-parts-mounted circuit boards and a variety ofelectronic parts.

1. Solder metal consisting essentially of 8.8 to 5.0 mass % of Zn, 0.05to 0 mass % of Bi and the balance of Sn and unavoidable impurities,wherein the solder metal is in the form of solder powder containingsolder particles having a particle size of 20 μm or less in an amountcontrolled to 30% or less in a number-basis particle-size distributionprofile.
 2. Solder metal consisting essentially of 8.5 to 6.5 mass % ofZn, 1.5 to 0.1 mass % of Bi and the balance of Sn and unavoidableimpurities, wherein the solder metal is in the form of solder powdercontaining solder particles having a particle size of 20 μm or less inan amount controlled to 30% or less in a number-basis particle-sizedistribution profile.
 3. Solder metal consisting essentially of 8.3 to7.5 mass % of Zn, 1.2 to 0.5 mass % of Bi and the balance of Sn andunavoidable impurities, wherein the solder metal is in the form ofsolder powder containing solder particles having a particle size of 20μm or less in an amount controlled to 30% or less in a number-basisparticle-size distribution profile.
 4. Soldering flux comprising ahydrocarbon compound having at least one primary, secondary or tertiaryCH bond and a halogen-containing and hydrogen-donating compound which isan organic halogenated compound, wherein the halogen-containing andhydrogen-donating compound is contained in an amount falling within arange of 0.02 to 20 mass % in terms of chlorine based on a total amountof the soldering flux.
 5. The soldering flux according to claim 4,wherein the hydrocarbon compound having at least one primary, secondaryor tertiary CH bond is one species selected from the group consisting oftetralin, tetraisobutylene, octahydroanthracene, 1 -α-naphthyl-1-n-butyl-hexadecene, 9,1 0-dihydro -9,10-diisobutylanthracene,n-octadecylbenzene, β-n-octadecyltetralin and polyisobutylene.
 6. Thesoldering flux according to claim 4, wherein the hydrocarbon compoundhaving at least one primary, secondary or tertiary CH bond is containedin an amount falling within a range of 0.01 to 20 mass%, based on atotal amount of the soldering flux.
 7. Solder paste comprising a soldermetal consisting essentially of 8.8 to 5.0 mass % of Zn, 0.05 to 0 mass% of Bi and the balance of Sn and unavoidable impurities and thesoldering flux according to claim
 4. 8. The solder paste according toclaim 7, wherein the solder metal is contained in an amount fallingwithin a range of 86 to 92 mass % and the soldering flux is contained inan amount falling within a range of 14 to 8 mass%, respectively, basedon a total amount of the solder paste.
 9. A method for producing aprinted circuit board, comprising the steps of applying the solder pasteaccording to claim 7 or 8 onto a circuit board, placing electronic partson portions to which the solder paste has been applied and heating thecircuit board so as to mount the electronic parts on the circuit board.10. Liquid flux for use in flow soldering, comprising the soldering fluxaccording to claim 4 that is diluted with a solvent. 11.Rosin-containing solder thread comprising a solder metal consistingessentially of 8.8 to 5.0 mass % of Zn, 0.05 to 0 mass % of Bi and thebalance of Sn and unavoidable impurities and the soldering fluxaccording to claim 4.